Multilayer optical bodies

ABSTRACT

Optical bodies, comprising: a plurality of first optical layers comprising a first polymer composition that comprises (i) a polyester portion having terephthalate comonomer units and ethylene glycol comonomer units, and (ii) a second portion corresponding to a polymer having a glass transition temperature of at least about 130° C.; and a plurality of second optical layers disposed in a repeating sequence with the plurality of first optical layers. Also disclosed are optical bodies comprising: (a) a plurality of first optical layers, each first optical layer being oriented; and (b) a plurality of second optical layers, disposed in a repeating sequence with the plurality of first optical layers, comprising a blend of polymethylmethacrylate and polyvinylidene fluoride. Methods of making the above-described optical bodies, and articles employing such optical bodies are also provided.

This patent application is a continuation of U.S. patent applicationSer. No. 10/215,791, filed Aug. 9, 2002, now U.S. Pat. No. 6,744,561,which is a continuation of U.S. patent application Ser. No. 09/444,756,filed Nov. 22, 1999, now U.S. Pat. No. 6,498,683, both of which areincorporated herein by reference. This patent application is also acontinuation-in-part of U.S. patent application Ser. No. 10/654,348,filed Sep. 2, 2003, which is a division of U.S. patent application Ser.No. 09/962,748, filed Sep. 25, 2001, and now issued as U.S. Pat. No.6,613,421, which is a division of U.S. patent application Ser. No.09/527,452, filed Mar. 17, 2000, and now issued as U.S. Pat. No.6,296,927, which is a division of U.S. patent application Ser. No.09/145,371, filed Sep. 2, 1998, and now issued as U.S. Pat. No.6,117,530, which is a continuation of U.S. patent application Ser. No.08/402,041, filed Mar. 10, 1995, and now issued as U.S. Pat. No.5,882,774, which is a continuation-in-part of U.S. patent applicationSer. No. 08/171,239, filed Dec. 21, 1993, abandoned, and is acontinuation-in-part of U.S. patent application Ser. No. 08/359,436,filed Dec. 20, 1994, abandoned

FIELD OF THE INVENTION

This invention relates to multilayer light-reflecting optical bodies. Inaddition, the invention relates to multilayer optical bodies thatreflect light over a range of wavelengths (e.g., mirrors, color mirroredfilms, IR reflective films, and UV reflective films).

BACKGROUND OF THE INVENTION

Polymeric films are used in a wide variety of applications. Oneparticular use of polymeric films is in mirrors which reflect light overa particular wavelength range. Such reflective films can be disposed,for example, behind a backlight in liquid crystal displays to reflectlight toward the display to enhance brightness of the display. Colorshifting films can be used in signage, packaging materials, etc. IRmirror films can be used, for example, to reduce solar heat loadentering a building or vehicle through its windows. Ultraviolet (UV)films can be used to protect other films or objects from UV light toprevent deleterious effects (e.g., photodegradation of a polymericfilm).

Coextrusion casting processes have been used to make multilayer opticalmirrors. Generally, however, cast films have a number of practicaldrawbacks. For example, cast films generally have low refractive indexdifferences between the high and low index materials and do notgenerally have matching refractive indices in the z-direction, limitingthe optical performance for a given number of layers. Because of thelimited optical power of such cast films, dyes and pigments alsotypically are used to enhance the color of color mirror films. Moreover,some cast films, particularly films made of noncrystalline materials,can also have limited thermal stability, dimensional stability,environmental stability and/or solvent resistance.

Coextrusion-orientation processes have been used to provide films withbetter optical performance due to the large refractive index differencebetween high and low index materials and the capability of matchingrefractive indices in the z-, or out-of-plane direction when at leastone of the materials is birefringent. One example of a previously formedfilm has high index layers formed of polyethylene naphthalate (PEN) andlow index layers of polymethyl methacrylate (PMMA). Orientation of PENincreases the refractive indices of the PEN layers and, therefore,increases the optical power of the PEN/PMMA films. PEN, however, is arelatively expensive material which is difficult to protect fromultraviolet radiation, and polyethylene terephthalate (PET), a lowerindex alternative to PEN, cannot easily be suitably oriented with PMMAdue to the difference in glass transition temperatures of thesematerials (about 84° C. for PET and about 106° C. for PMMA).

SUMMARY OF THE INVENTION

In aspect, the present invention provides an optical body, comprising:(a) a plurality of first optical layers, each first optical layer beingoriented and comprising a polyester having terephthalate comonomer unitsand ethylene glycol comonomer units and having a glass transitiontemperature less than or equal to about 90° C.; and (b) a plurality ofsecond optical layers disposed in a repeating sequence with theplurality of first optical layers, each second optical layer comprisinga polymer composition; the optical body being configured and arranged toreflect at least a portion of light over at least one wavelength region.

In another aspect, the invention provide an optical body, comprising: aplurality of first optical layers, each first optical layer beingoriented and comprising a first polymer composition, the first polymercomposition comprising:

-   -   (i) a polyester portion having terephthalate comonomer units and        ethylene glycol comonomer units, and    -   (ii) a second portion corresponding to a polymer having a glass        transition temperature of at least about 130° C.; and

a plurality of second optical layers disposed in a repeating sequencewith the plurality of first optical layers, each second optical layercomprising a second polymer composition.

In yet another aspect, the invention provides an optical body,comprising:

(i) a plurality of first optical layers, each first optical layer beingoriented and comprising a polyester having terephthalate comonomer unitsand ethylene glycol comonomer units; and

(ii) a plurality of second optical layers disposed in a repeatingsequence with the plurality of first optical layers, each second opticallayer comprising a polymer composition, the polymer composition having aglass transition temperature of less than or equal to about 90° C. andcomprising a polymer selected from the group consisting of polyacrylatesand aliphatic polyolefins.

In still another aspect, the invention provides an optical body,comprising: (a) a plurality of first optical layers, each first opticallayer being oriented; and (b) a plurality of second optical layersdisposed in a repeating sequence with the plurality of first opticallayers, each second optical layer comprising a blend ofpolymethylmethacrylate and polyvinylidene fluoride; the optical bodybeing configured and arranged to reflect at least a portion of lightover at least one wavelength region.

Methods of making the above-described optical bodies, and articlesemploying such optical bodies are also provided.

The above summary of the present invention is not intended to describeeach disclosed embodiment or every implementation of the presentinvention. The Figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional diagram of a first embodiment of amultilayer optical body according to the invention;

FIG. 2 is a schematic cross-sectional diagram of a second embodiment ofa multilayer optical body according to the invention;

FIG. 3 is a graph of the transmission spectra for polycarbonate,polyethylene terephthalate, and polyethylene naphthalate;

FIG. 4 is the transmission spectrum for the optical film of Example 1;

FIG. 5 is the transmission spectrum for the optical film of Example 2;

FIG. 6 is the transmission spectrum for the optical film of Example 3;

FIG. 7 is the transmission spectrum for the optical film of Example 4;

FIG. 8 is the transmission spectrum for the optical film of Example 5;

FIG. 9 is the transmission spectrum for the optical film of Example 6;

FIG. 10 is the transmission spectrum for the optical film of Example 14;

FIG. 11 is the transmission spectrum for the optical film of Example 15;

FIG. 12 is the transmission spectrum for the optical film of Example 16;

FIG. 13 is the transmission spectrum for the optical film of Example 17;

FIG. 14 is the transmission spectrum for the optical film of Example 18;

FIG. 14 is the transmission spectrum for the optical film of Example 19;

FIG. 16 is the transmission spectrum for the optical film of Example 20;

FIG. 17 is the transmission spectrum for the optical film of Example 21;

FIG. 18 is the transmission spectrum for the optical film of Example 22;

FIG. 19 is a graph of the comparison of reflectivity between opticalfilms with PMMA/PVDF and PMMA as second optical layers;

FIG. 20 is a graph of reflectivity for optical films with PMMA/PVDF andPMMA as second optical layers, at the same crossweb position;

FIG. 21 is a graph of reflectivity for optical films with PMMA/PVDF andPMMA as second optical layers and with similar bandwidths;

FIG. 22 is a comparison of a* as a function of wavelength for opticalfilms with PMMA/PVDF and PMMA as second optical layers; and

FIG. 23 is a comparison of b* as a function of wavelength for opticalfilms with PMMA/PVDF and PMMA as second optical layers.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is generally directed to light-reflectingmultilayer optical bodies (such as multilayer optical films) and theirmanufacture, as well as the use of the multilayer optical bodies aspolarizers and mirrors and in devices. These multilayer optical bodiesinclude multilayer optical films, methods of making and using thesemultilayer optical films, and articles incorporating the multilayeroptical films. The multilayer optical bodies reflect light over awavelength range (e.g., all or a portion of the visible, IR, or UVspectrum). The multilayer optical bodies are typically coextruded andoriented multilayer structures that differ from previous optical bodies,at least in part, due to the selection of materials which can provideprocessing, economic, optical, mechanical, and other advantages. Whilethe present invention is not so limited, an appreciation of variousaspects of the invention will be gained through a discussion of theexamples provided below.

The term “birefringent” means that the indices of refraction inorthogonal x, y, and z directions are not all the same. For the polymerlayers described herein, the axes are selected so that x and y axes arein the plane of the layer and the z axis is normal to the plane of thelayer and typically corresponds to the thickness or height of the layer.For an oriented polymer, the x-axis is generally chosen to be thein-plane direction with the largest index of refraction, which typicallycorresponds to one of the directions in which the optical body isoriented (e.g., stretched).

The term “in-plane birefringence” is the absolute value of thedifference between the in-plane indices (n_(x) and n_(y)) of refraction.

The term “polymer” will be understood, unless otherwise indicated, toinclude polymers and copolymers (i.e., polymers formed from two or moremonomers including terpolymers, etc.), as well as polymers or copolymerswhich can be formed in a miscible blend by, for example, coextrusion orreaction, including transesterification. Block, random, graft, andalternating copolymers are included, unless indicated otherwise.

All birefringence and index of refraction values are reported for 632.8nm light unless otherwise indicated.

Multilayer Optical Films

FIGS. 1 and 2 illustrate multilayer optical bodies 10 (e.g., multilayeroptical films) that can be used as, for example, a mirror, a polarizer,an IR film, a UV film, or a color-shifting film. The optical bodies 10include one or more first optical layers 12, one or more second opticallayers 14, and one or more non-optical layers 18. The non-optical layers18 can be disposed on a surface of the multilayer optical body as a skinlayer (FIG. 1) or disposed between optical layers (FIG. 2). The firstand second optical layers and, optionally, the non-optical layers, ifany, are coextruded and oriented by, for example, stretching.Orientation typically significantly enhances the optical power (e.g.,reflectivity) of the multilayer optical bodies due to birefringence ofthe first or second optical layers or both.

Such multilayer optical bodies include multilayer optical films that aresuited to applications such as, for example, (i) reflective polarizersused in laptop and palmtop computer displays, cellular phone, pager, andwatch displays, (ii) mirror films used in enhanced lighting, medical,and horticultural applications, (iii) color shifting films fordecorative and security applications, (iv) IR reflective films used asheat management films in fields such as the automotive, horticultural,optoelectronic filtration, and architectural fields, and (v) UVreflective films used, for example, to protect other films and objectsfrom UV radiation.

The optical layers 12, 14 are typically interleaved to form a stack 16of layers, optionally, with one or more of the non-optical layers 18included within or as a skin layer of the stack. Typically the opticallayers 12, 14 are arranged as alternating pairs, as shown in FIG. 1, toform a series of interfaces between layers with different opticalproperties. The optical layers 12, 14 are typically no more than 1 μmthick and can have a thickness of 400 nm or less. The optical layers canhave the same thicknesses. Alternatively, the multilayer optical bodycan include layers with different thicknesses to increase the reflectivewavelength range.

Although FIG. 1 shows only six optical layers 12, 14, multilayer opticalbodies 10 can have a large number of optical layers. Examples ofsuitable multilayer optical bodies include those having about 2 to 5000optical layers. Generally, multilayer optical bodies have about 25 to2000 optical layers and typically about 50 to 1500 optical layers orabout 75 to 1000 optical layers. It will be appreciated that, althoughonly a single stack 16 is illustrated in FIG. 1, the multilayer opticalbody 10 can be made from multiple stacks that are subsequently combinedto form the optical body 10.

Additional sets of optical layers, similar to the first and secondoptical layers 12, 14, can also be used in the multilayer optical body10. The design principles disclosed herein for the sets of first andsecond optical layers can be applied to any additional sets of opticallayers. In addition, different repeating patterns of optical layers canbe used (e.g., “ABCBA . . . ”, where A, B, and C are optical layers withdifferent compositions). Some such patterns as set forth in U.S. Pat.No. 5,360,569, which is incorporated herein by reference.

The transmission and reflection characteristics of the multilayeroptical bodies are based on coherent interference of light caused by therefractive index difference between the first and second optical layersand the thicknesses of the first and second optical layers. When thein-plane indices of refraction differ between the first and secondoptical layers, the interface between adjacent first and second opticallayers forms a reflecting surface. The reflective power of the interfacedepends on the square of the difference between the in-plane indices ofrefraction of the first and second optical layers (e.g., (n₁₀−n₂₀)²,where n₁₀ is an in-plane refractive index of the first optical layersand n₂₀ is an in-plane refractive index of the second optical layers).

In mirror applications, the multilayer optical body typically includesfirst and second optical layers where both in-plane refractive indicesdiffer substantially (e.g., differ by at least 0.04 and, often, by atleast 0.1) between the layers (i.e., n_(1x)≠n_(2x) and n_(1y)≠n_(2y),where n_(1x) and n_(1y) are the in-plane refractive indices of the firstoptical layers and n_(2x) and n_(2y) are the in-plane refractive indicesof the second optical layers). In polarizer applications, the multilayeroptical body typically includes first and second layers where one of thein-plane refractive indices differs substantially between the layers andthe other in-plane refractive index is substantially similar (e.g.,n_(1x)≠n_(2x) and n_(1y)≈n_(2y)). Preferably, the substantially similarin-plane refractive indices differ by no more than about 0.04. Forpolarizer applications, the in-plane birefringence of the first opticallayers is typically at least about 0.05, preferably at least about 0.15,and more preferably at least about 0.2.

The first optical layers 12 are made using birefringent polymers(preferably, polymers with positive birefringence) that are uniaxially-or, preferably, biaxially-oriented to increase the in-plane refractiveindex (or indices) of the first optical layers, thereby increasing thedifference between the refractive indices of the first and secondlayers. In some embodiments, the second optical layers 14 are polymerlayers that are birefringent (preferably, negatively birefringent) anduniaxially- or biaxially-oriented. In other embodiments, the secondoptical layers 14 are polymer layers having an isotropic index ofrefraction (e.g., substantially the same index of refraction in alldirections) that is typically different from one or both of the in-planeindices of refraction of the first optical layers 12. Although, thepresent invention will be exemplified using optical bodies 10 withsecond optical layers 14 that have an isotropic index of refraction, theprinciples and examples described herein can be applied to multilayeroptical bodies with second optical layers 14 that are birefringent.

The first optical layers 12 can be made birefringent by, for example,stretching the first optical layers 12 in a desired direction ordirections. For example, the first optical layers 12 can bebiaxially-oriented by stretching in two different directions. Thestretching of optical layers 12 in the two directions can result in anet symmetrical or asymmetrical stretch in the two chosen orthogonalaxes. Symmetrical stretching in two directions can yield in-planerefractive indices that are substantially similar (e.g., differ by nomore than 0.4). As an alternative to stretching in two directions, thefirst optical layers 12 can be uniaxially-oriented by, for example,stretching the layers in a single direction. A second orthogonaldirection may be allowed to neck (e.g., decrease in length, width, orthickness) into some value less than its original length. The directionof stretching typically corresponds to either in-plane axis (e.g. the xor y axis), however, other directions can be chosen.

Typically, the highest reflectivity for a particular interface betweenfirst and second optical layers occurs at a wavelength corresponding totwice the combined optical thickness of the pair of optical layers 12,14. The optical thickness describes the difference in path lengthbetween light rays reflected from the lower and upper surfaces of thepair of optical layers. For light incident at 90 degrees to the plane ofthe optical film (normally incident light), the optical thickness of thetwo layers is n₁d₁+n₂d₂ where n₁, n₂ are the in-plane indices ofrefraction of the two layers and d₁, d₂ are the thicknesses of thecorresponding layers. The equation λ/2=n₁d₁+n₂d₂ can be used to tune theoptical layers for normally incident light. At other angles, the opticaldistance depends on the distance traveled through the layers (which islarger than the thickness of the layers) and the indices of refractionfor at least two of the three optical axes of the layer. The opticallayers 12, 14 can each be a quarter wavelength thick or the opticallayers 12, 14 can have different optical thicknesses, as long as the sumof the optical thicknesses is half of a wavelength (or a multiplethereof). A multilayer optical body having more than two optical layerscan include optical layers with different optical thicknesses to providereflectivity over a range of wavelengths. For example, a multilayeroptical body can include pairs or sets of layers that are individuallytuned to achieve optimal reflection of normally incident light havingparticular wavelengths or may include a gradient of layer pairthicknesses to reflect light over a larger bandwidth.

These multilayer optical bodies can be designed to reflect one or bothpolarizations of light over at least one bandwidth. The layerthicknesses and indices of refraction of the optical stacks within theoptical bodies can be controlled to reflect at least one polarization ofspecific wavelengths of light (at a particular angle of incidence) whilebeing transparent over other wavelengths. Through careful manipulationof these layer thicknesses and indices of refraction along the variousoptical body axes, the multilayer optical body of the present inventionmay be made to behave as mirrors or polarizers over one or more regionsof the spectrum.

For example, the optical bodies can be designed to reflect light oversubstantially all of the visible light region (about 400 to 750 nm) toform a visible mirror. The visible mirror may be a cold mirror,reflecting only the visible wavelengths of light and transmitting theIR, or it may be a broadband mirror that reflects both the visible andIR wavelengths. Visible mirrors are described, for example, in U.S. Pat.No. 5,882,774 and WO 97/01774, and a cold mirror is described, forexample, in U.S. Pat. Nos. 5,339,198 and 5,552,927, all of which areincorporated herein by reference. For cold mirrors, the typical opticallayer thickness is in the range of 100 to 200 nm. For visible/IRmirrors, the typical optical layer thickness is in the range of 100 to600 nm (for a ¼ wavelength design).

Another embodiment is an optical body that reflects at least a portionof infrared (IR) light. To reflect light in the region from about 750 to1200 nm, the layers have optical thicknesses ranging from about 185-300nm (¼ the wavelength of the light desired to be reflected). For example,the optical bodies of the present invention can be tuned to reflect bothpolarizations of light in at least a portion of the IR region of thespectrum while being transparent over other portions of the spectrum.This type of optical body can be used as an IR film to, for example,reflect solar energy from, for example, windows of buildings andautomobiles. Preferably, IR films for these uses transmit a largeportion of the visible light and, more preferably, have substantiallyuniform transmission spectra over the visible range to avoid theappearance of color. Further description of IR films and examples offilm configurations are presented in WO 97/01778, WO 99/36808, and U.S.Pat. No. 5,360,659, all of which are incorporated herein by reference.

Yet another embodiment is a multilayer optical body that reflects lightover only a portion of the visible range. These optical bodies can beused as color shifting films, because as viewing angle changes, thewavelength region of reflection also changes. Further description ofcolor changing films, principles of operation, and examples of filmconfigurations are presented in WO 99/36257 and WO 99/36258, both ofwhich are incorporated herein by reference. These optical bodies can betailored to exhibit a sharp bandedge at one or both sides of at leastone reflective bandwidth, thereby giving a high degree of colorsaturation at acute angles, if desired, as described in WO 99/36809,incorporated herein by reference.

First Optical Layers

The first optical layers 12 are typically orientable films ofpolyethylene naphthalate (PEN), polyethylene terephthalate (PET), orcopolymers or blends thereof. Examples of suitable copolymers aredescribed in, for example, WO 99/36262, and in co-pending U.S. patentapplication Ser. No. 09/399,531, both of which are incorporated hereinby reference. Other suitable materials for the first optical layersinclude other polyesters, including for example, polycarbonate,polyarylate, and naphthalate and terephthalate-containing polymers, suchas, for example, polybutylene naphthalate (PBN), polypropylenenaphthalate (PPN), polybutylene terephthalate (PBT), and polypropyleneterephthalate (PPT), and blends or copolymers of any of the above witheach other or with non-polyester polymers.

Preferred properties of the material used for the first optical layersinclude: 1) birefringence (preferably, positive birefringence), 2)thermal stability, 3) processing temperatures compatible with thematerials of the second optical layers, 4) UV stable or protectable, 5)high clarity (e.g., high transmission and low absorption), 6) a glasstransition temperature that is compatible with the second optical layersto provide strain-induced birefringence, 7) a range of viscosities topermit viscosity matching with the materials of the second opticallayers, 8) good interlayer adhesion with the second optical layers, 9)low dispersion, 10) good z-index matching with the second opticallayers, and 11) drawability (e.g., the ability to be stretched). Otherfactors include cost and commercial availability.

PEN, PET, and copolymers and blends of PEN and PET, as well as the otherpolymers listed above, can be made birefringent by, for example,stretching the first optical layers 12 in a desired direction ordirections. Orientation is typically accomplished at a temperature abovethe glass transition temperature of the polymer, although somecopolymers with low crystallinity can be oriented at or below the glasstransition temperature as described in, for example, co-pending U.S.patent application Ser. No. 09/399,531, incorporated herein byreference.

Polyethylene naphthalate (PEN) can have a relatively large birefringenceupon orientation. For example, uniaxial orientation of PEN can raise therefractive index of PEN in the orientation direction from 1.64 to 1.88.Biaxial orientation of PEN can raise the refractive index of PEN in theorientation directions from 1.64 to 1.75, while the z index ofrefraction decreases to 1.49, giving a birefringence of 0.24 to 0.26between the in-plane and z-axis refractive indices.

Uniaxial orientation of polyethylene terephthalate (PET) can raise therefractive index of PET in the orientation direction from 1.57 to 1.69.Biaxial orientation of PET can raise the refractive index of PET in theorientation directions from 1.57 to 1.65, while the z index ofrefraction decreases to 1.50, giving a birefringence of 0.13 to 0.15between the in-plane and z-axis refractive indices.

The amount of birefringence and the amount of change in refractive indexobtained for these polymers depends on a variety of factors including,for example, the draw ratio, the orientation temperature, and whetherthe polymer is uniaxially or biaxially oriented. Typically, the largerthe draw ratio, the larger the change in refractive index. However, theachievable draw ratio can be limited by the orientation temperature.

Typically, for relatively crystalline materials, the orientationtemperature is above the glass transition temperature. Generally, thecloser that the orientation temperature is to the glass transitiontemperature, the lower the achievable draw ratio because the polymerexhibits excessive strain hardening when drawn and can crack or formmicrovoids. However, in general, the closer that the orientationtemperature is to the glass transition temperature, the larger thechange in refractive index for a given draw ratio. Thus, drawing thepolymer at a temperature that is substantially above (e.g., 20° C. or30° C.) the glass transition temperature of the polymer will typicallyresult in significantly less change in the refractive index for a givendraw ratio. Thus, a balance is required between draw ratio andorientation temperature to achieve a desired refractive index change.

Material selection can influence the optical and physical properties ofthe multilayer optical body. Polyesters, like PEN and PET, includecarboxylate and glycol subunits and can be generated by, for example,(a) reaction of carboxylate monomers with glycol monomers or (b)transesterification. Each carboxylate monomer has two or more carboxylicacid or ester functional groups and each glycol monomer has two or morehydroxy functional groups. Polyesters can be formed using a single typeof carboxylate monomer or two or more different types of carboxylatemonomers. The same applies to the glycol monomers. Also included withinthe term “polyester” are polycarbonates which are derived from thereaction of glycol monomers with esters of carbonic acid.

The properties of a polymer layer or film vary with the particularchoice of monomers. PEN includes carboxylate subunits formed from2,6-naphthalene dicarboxylic acid or esters thereof and PET includescarboxylate subunits formed from terephthalic acid or esters thereof.Suitable carboxylate comonomers for forming the carboxylate subunits ofcopolyesters of PEN and PET include, for example, 2,6-naphthalenedicarboxylic acid and isomers thereof; terephthalic acid; isophthalicacid; phthalic acid; azelaic acid; adipic acid; sebacic acid; norbornenedicarboxylic acid; bi-cyclooctane dicarboxylic acid; 1,6-cyclohexanedicarboxylic acid and isomers thereof; t-butyl isophthalic acid;tri-mellitic acid; sodium sulfonated isophthalic acid; 2,2′-biphenyldicarboxylic acid and isomers thereof; and lower alkyl esters of theseacids, such as methyl or ethyl esters. The term “lower alkyl” refers, inthis context, to C1-C10 straight-chained or branched alkyl groups.

Both PEN and PET include glycol subunits formed using ethylene glycol.Suitable glycol comonomers for forming glycol subunits of copolyestersof PEN and PET include propylene glycol; 1,4-butanediol and isomersthereof; 1,6-hexanediol; neopentyl glycol; polyethylene glycol;diethylene glycol; tricyclodecanediol; 1,4-cyclohexanedimethanol andisomers thereof; norbornanediol; bicyclo-octanediol; trimethylolpropane; pentaerythritol; 1,4-benzenedimethanol and isomers thereof;bisphenol A; 1,8-dihydroxy biphenyl and isomers thereof; and1,3-bis(2-hydroxyethoxy)benzene.

Multilayer optical bodies can also be formed using polyethyleneterephthalate (PET). PET has a lower refractive index than PEN, but PETis much less expensive (currently about one eighth the cost of PEN).Despite the lower refractive index of PET, the ratio of optical power(the square of the difference between the in-plane refractive indices ofthe first and second optical layers) and cost currently favors PET overPEN and other materials, such as polycarbonate.

Moreover, PET and PET-containing films can be more easily protected fromultraviolet (UV) degradation than PEN without introducing color into theUV-protected optical body. FIG. 3 illustrates the transmission spectraof PEN, PET, and polycarbonate. PEN absorbs light at 380 nm with anabsorption tail extending into the visible region of the spectrum toabout 410 nm. A UV protective coating or additive for an optical bodymade using PEN would typically extend into the visible range, which maygive the optical body a yellowish color (due to absorption of bluelight).

On the other hand, PET absorbs light at 320 nm with a tail extending to370 nm. Thus, a UV protecting coating or additive would not need toextend into the visible range. This ability is particularly importantwhen preparing multilayer optical bodies that are designed to reflect IRlight and transmit visible light (e.g., solar reflective films forbuilding and automobile windows) or optical bodies designed to reflectonly a particular bandwidth in the visible range and transmitting allother light.

Furthermore, the lower refractive index and lower dispersion of PET canbe advantageous in at least some respects. For example, clarity ortransparency of an IR film can be essential, particularly forarchitectural and automotive applications. Because of the lowerrefractive index difference of PET relative to PEN, a PET-basedmultilayer optical body can have less iridescence and spectralnonuniformities because the optical spectrum is a weaker function ofviewing angle. Wider IR bandwidth can also be achieved because the lowerrefractive index and lower dispersion in PET permits the positioning ofhigher order harmonics at higher wavelengths.

Another advantage, when the skin layers are made of the same material asthe first optical layers, is that PET, because of its lower refractiveindex, will typically have lower surface reflection because of the lowerrefractive index mismatch between the air (or other material)/PETinterface. This is particularly useful in automotive applications, wherepoly(vinyl butyrate) (PVB) is used to generate safe shattering windows.The refractive index of PVB is closer to PET than PEN, thereby reducingglare.

Suitable PET-containing multilayer optical bodies can be formed in avariety of configurations. Particularly useful PET-based materialsinclude PET or PET copolymers or blends that have a glass transitiontemperature of no more than about 90° C., or of no more than about 80°C. or 70° C. Typically, the most useful of these PET-based materialswill be free, or substantially free, of naphthalene dicarboxylate (NDC)monomers. In such constructions, the material for the second opticallayers generally will also include a material having a glass transitiontemperature of no more than about 90° C. Among the materials forsuitable second optical layers are polyacrylates and aliphaticpolyolefins, including blends of these polymers with other materials andcopolymers. Alternatively, the first optical layers can be formed usinga copolymer or blend of PET that is also substantially free of NDCmonomer and that has a glass transition temperature of at least about100° C. or at least 120° C. In such constructions, the material for thesecond optical layers generally will also include a material having aglass transition temperature of at least about 100° C.

As an alternative, the glass transition temperature of PET can be raisedby combining PET with a second polymer that has a higher glasstransition temperature. The combination of PET and the second polymercan include miscible blending to form a polymer blend or reactiveblending (by, for example, transesterification) to form a copolymer. Forexample, PET can be blended with a second polymer that has a glasstransition temperature of 130° C. or higher or a second polymer with aglass transition temperature of 160° C. or higher, or even a secondpolymer with a glass transition temperature of 200° C. or higher.Examples of suitable second polymers include, for example, PEN(T_(g)=130° C.), polycarbonate (T_(g)=157° C.), polyarylate (T_(g)=193°C.), or polyetherimide (T_(g)=218° C.).

Alternatively, the monomer materials of PET, e.g., terephthalic acid andethylene glycol, can be copolymerized with the monomer materials thatcorrespond to a second polymer having a higher glass transitiontemperature, such as PEN, polycarbonate, and polyarylate, to formcopolymers. For example, PET can be copolymerized with monomer materialsthat correspond with a second polymer that has a glass transitiontemperature of 130° C. or higher or a second polymer with a glasstransition temperature of 160° C. or higher, or even a second polymerwith a glass transition temperature of 200° C. or higher.

Other copolymers of PET can also be used, including those incorporating(i) carboxylate monomer materials, such as, for example, isophthalicacid; phthalic acid; azelaic acid; adipic acid; sebacic acid; norbornenedicarboxylic acid; bi-cyclooctane dicarboxylic acid; 1,6-cyclohexanedicarboxylic acid and isomers thereof; t-butyl isophthalic acid;tri-mellitic acid; sodium sulfonated isophthalic acid; 2,2′-biphenyldicarboxylic acid and isomers thereof; and lower alkyl esters of theseacids, such as methyl or ethyl esters; and (ii) glycol monomermaterials, such as, for example, propylene glycol; 1,4-butanediol andisomers thereof; 1,6-hexanediol; neopentyl glycol; polyethylene glycol;diethylene glycol; tricyclodecanediol; 1,4-cyclohexanedimethanol andisomers thereof; norbornanediol; bicyclo-octanediol; trimethylolpropane; pentaerythritol; 1,4-benzenedimethanol and isomers thereof;bisphenol A; 1,8-dihydroxy biphenyl and isomers thereof; and1,3-bis(2-hydroxyethoxy)benzene.

At least some of these materials, such as the copolymers of PET and PEN,have lower processing temperatures than PET and can be more effectivelyused with second optical layer materials, such as EVA, that degrade atPET processing temperatures.

Second Optical Layers

The second optical layers 14 can be prepared with a variety of opticaland physical properties depending, at least in part, on the desiredoperation of the film 10. Preferred properties of the second opticallayers include, for example, 1) isotropic or negative birefringence, 2)thermal stability, 3) processing temperatures compatible with thematerials of the first optical layers, 4) UV stable or protectable, 5)high clarity (e.g., high transmission and low absorption), 6) a glasstransition temperature that is compatible with the first optical layersto provide strain-induced birefringence, 7) a range of viscosities topermit viscosity matching with the materials of the first opticallayers, 8) good interlayer adhesion with the first optical layers, 9)low dispersion, 10) good z-index matching with the first optical layers,and 11) drawability (e.g., the ability to be stretched). Other factorsinclude cost and commercial availability.

In some embodiments, the second optical layers 14 are made of a polymermaterial that does not appreciably optically orient when stretched underconditions that are used to orient the first optical layers 12. Suchlayers are particularly useful in the formation of reflective opticalbodies, because they allow the formation of a stack 16 of layers by, forexample, coextrusion, which can then be stretched to orient the firstoptical layers 12 while the second optical layers 14 remain relativelyisotropic (e.g., an in-plane birefringence of 0.05 or less). In otherembodiments, the second optical layers 14 are orientable and are,preferably, negatively birefringent (when the first optical layers arepositively birefringent) so that the in-plane refractive indicesdecrease with orientation.

There are a variety of considerations in the selection of the materialsfor the first and second optical layers. The importance of theseconsiderations typically depends on the desired optical properties anduses for the optical bodies. One consideration is the glass transitiontemperature of the second optical layers. Typically, the materials ofthe first and second optical layers are selected so that the glasstransition temperature of the second optical layers is not substantiallyhigher than the glass transition temperature of the first opticallayers. More preferably, the glass transition temperature of the secondoptical layers is equal to or less than the glass transition temperatureof the first optical layers. If the glass transition temperature of thesecond optical layers is too high, orientation of the first opticallayers at a suitable orientation temperature near the glass transitiontemperature of the first optical layers can cause excessive strainhardening in the second optical layers. This can diminish the opticalquality of the second optical layers by, for example, introducing cracksor microvoids. The glass transition temperature of an optical layer isdefined as the glass transition temperature of the composition that isused to form the optical layer and not the glass transitiontemperature(s) of the individual components.

Another consideration is the difference in the z-axis refractive indicesbetween the first and second optical layers. When the z-axis refractiveindices of the two layers are equal, the reflectance of p-polarizedlight does not depend on the incident angle of light. This feature canbe useful when reflectance uniformity over a range of viewing angles isdesired. In such embodiments, the difference in z-axis refractiveindices between the first and second optical layers is preferably nomore than about 0.04 and, by selection of materials can be made no morethan about 0.02 or no more than about 0.01.

Another consideration is the decomposition temperature of the polymer(s)selected for use in the second optical layers. Typical coextrusionprocessing temperatures for PEN and PET are above about 250° C.Degradation of the components of the second optical layers can producedefects in the optical body, such as, for example, discoloration andregions of gel formation. Materials that do decompose at the processingtemperatures can still be used if the decomposition does not result inunacceptable properties.

The second optical layers 14 can be made using a variety of polymericcompositions. The description of suitable polymers with respect toparticular optical body configurations is provided below.

Multilayer optical bodies have been made using PEN as the first opticallayer and polymethyl methacrylate (PMMA) as the second optical layer.While PMMA has been found to have a number of properties such as clarityand viscosity that make it useful as a low refractive index material formultilayer films, some characteristics of PMMA have limited itsusefulness. In particular, the relatively high refractive index (n=1.49)and glass transition temperature (T_(g)=106° C.) have limited the choiceof materials that can be used in the first optical layers. For example,while PMMA can be used with PEN, its glass transition temperature makesit unsuitable in many applications for processing with PET, and itsrefractive index is not sufficiently low to create the difference inrefractive indices with PET that is desired for many applications. Theuse of PET instead of PEN can be desirable due to the susceptibility ofPEN to UV degradation and the resultant need to provide additional UVprotection when PEN is used.

One aspect of this invention utilizespolymethylmethacrylate/polyvinylidene fluoride blends (PMMA/PVDF) in thesecond optical layers as low refractive index materials. The PMMA/PVDFblends are particularly useful with polyester high refractive indexmaterials, for example aromatic polyesters such as polyethyleneterephthalate (PET) and polyethylene naphthalate (PEN), as well asblends and copolymers thereof.

The polymers used in multilayer optical films should be clear so thatlight is not lost by scattering or absorption. Many applications involvemultiple interactions between light and the optical film, which magnifythe adverse affects of scattering and absorption. Optical polymers suchas PMMA are considered sufficiently clear for most purposes, withtransmission in the visible region of the spectrum at 92%. PVDF has atransmission of 96%. PMMA/PVDF miscible blends have higher transmission(clarity) than PMMA.

PMMA/PVDF miscible blends have a lower refractive index than PMMA(n=1.49) due to the low index of PVDF (n=1.42). The larger indexdifference results in greater optical power in the multilayer film. Therefractive index for a PMMA/PVDF(60/40) (by weight) miscible blend isabout 1.458. The larger index difference provided by the PMMA/PVDF blendrelative to PMMA also results in a significant dampening of color leaksas well as higher reflectivity.

Multilayer films using either PEN or PET require high coextrusiontemperatures (greater than or equal to about 250° C.) due to the highmelting points of these polyesters. Second optical layers that are notthermally stable can cause flow instabilities in the multilayer film dueto viscosity loss associated with degradation. Degradation products alsomay result in point defects or discoloration in the optical film.PMMA/PVDF miscible blends are more thermally stable than PMMA.

Use of PMMA/PVDF miscible blends in place of PMMA in the second opticallayers permits a wider variety of materials for the first optical layersbecause of the lower glass transition temperature (T_(g)) of the blends.For example, PMMA/PVDF blends enable the use PET or copolymers of PET inthe first optical layers of multilayer optical bodies. PET has severaladvantages relative to PEN in multilayer optical bodies including easierUV protection and PET is a less expensive raw material. When a castmultilayer optical body is stretched near the T_(g) of the high indexmaterial, the orientation near the T_(g) results in a high degree ofstress birefringence and thus a high refractive index for PEN (or PET)in the direction of stretch. PEN:PMMA multilayer films are orientablenear the T_(g) of PEN because the T_(g) of PMMA (T_(g)=106° C.) is lowerthan that of PEN (T_(g)=123° C.). PET:PMMA are not orientable near theT_(g) of PET because of the high T_(g) of PMMA relative to PET(T_(g)=84° C.). Blending as little as 25% (by weight) PVDF to PMMAdepresses the T_(g) of PMMA, PMMA/PVDF(75/25) (T_(g)=72° C.), enablingthe use of PET in the first optical layers.

Thus, by blending PVDF with PMMA, a low refractive index material (forthe second optical layers) with improved properties is achieved. Suchblends have a lower refractive index and a lower glass transitiontemperature as compared to PMMA, while at least maintaining suitableperformance in properties such as clarity, viscosity, thermal stabilityand interlayer adhesion. In particular, the blends, when coextruded withPEN or PET or blends or copolymers thereof) as the high refractive indexmaterial (for the first optical layers), exhibit properties such asexcellent clarity (e.g. transmission>90%), low refractive index(n≦1.49), viscosity similar to that of the high refractive indexmaterial, thermal stability at temperatures greater than 250° C., glasstransition temperature (T_(g)) below that of the high refractive indexmaterial; and good interlayer adhesion with the high refractive indexmaterial.

As a result, the PMMA/PVDF blends can be used with conventional highrefractive index materials such as PEN and achieve improved optical orphysical properties. For example, due to the lowered refractive index,the use of the PMMA/PVDF blends can require fewer layers to achieve thesame optical effects as a corresponding product made with PMMA in thesecond optical layers, or can provide enhanced effects when the samenumber of layers is used. The PMMA/PVDF blends also can be used withhigh refractive index materials such as PET and copolymers of PET thatpreviously have not been found suitable for practical application inmultilayer optical bodies. Further, the improved optical or physicalproperties are advantageous in permitting less strict control overprocessing conditions in forming the multilayer films while stillachieving the desired performance. This permits more practical andcost-effective manufacturing of the multilayer optical bodies.

In particular, some of the advantages seen with the use of the PMMA/PVDFblends as low refractive index materials versus PMMA alone include (1)higher reflectivity—the greater difference in refractive index yieldsgreater reflectivity for a given number of layers; (2) reduction ofspectral non-uniformity or iridescence (which often results fromprocessing difficulties such as layer non-uniformity)—again due to thelarger difference in refractive index—thereby providing optical benefitsand processing benefits; (3) increased efficiency in applicationsrequiring multiple reflections; (4) wider processing tolerances such ascrossweb caliper uniformity requirements and lack of optical packetoverlap; (5) thinner film requirements for a given reflectivity; (6)more options for high refractive index materials; (7) improvedperformance in thermoforming, embossing and the like due to lower T_(g)and (8) easier UV protection because high refractive index materialswith reduced levels of PEN can be used.

The particular PMMA and PVDF used in the blends to provide a lowrefractive index material are not limited so long as the materials aresufficiently miscible with each other and the resultant blend can becoextruded with the high refractive index material to form themultilayer film. For example, PMMA sold under the designations Perspex™CP80 and CP82 by ICI Americas, Inc. (Wilmington, Del.) and PVDF soldunder the designation Solef™ 1008/0001 by Solway are useful with PET andPEN high refractive index materials.

The amount of PVDF used in the blends is typically not more than about40% by weight (i.e. a 60/40 PMMA/PVDF blend). With higher levels ofPVDF, the miscibility of the PMMA and PVDF tends to deteriorate, therebycausing losses in clarity. In general, it is desirable to use PVDF inthe blends in an amount as high as possible in order to increase thebenefit in reductions in refractive index and glass transitiontemperature. However, smaller amounts can be used when it is desired tofine tune the composition to provide particular optical or physicalproperties for certain applications. For example, a 75/25 blend provideshighly desirable physical and optical properties for use with highrefractive index materials such as PEN, PET and mixtures or copolymersthereof.

As indicated above, polymethyl methacrylate (PMMA) is a useful materialfor forming the optical bodies. However, the glass transitiontemperature of PMMA is about 106° C., which is significantly higher thanthe glass transition temperature of PET (T_(g) is about 84° C.). Anoptical body with first optical layers of PET and second optical layersof PMMA would be oriented at a temperature above the glass transitiontemperature of PMMA, significantly reducing the refractive index changeexpected for orientation of PET.

As described above, the blending of polyvinylidene fluoride (PVDF) withPMMA reduces the glass transition temperature of the blended polymers.Preferably, the blend includes about 20 to 40 wt. % PVDF and 60 to 80wt. % PMMA. Below about 20 wt. % PVDF, the glass transition temperatureis above that of PET, although these blends are still acceptable forsome applications. Above about 40 wt. %, PVDF crystallizes. The additionof PVDF to the second optical layers can also enhance other properties,such as, for example, solvent resistance.

As another option, copolymers of PMMA can be made using comonomers thatdepress the glass transition temperature of the copolymer below theglass transition temperature of PMMA. Suitable comonomers include otheracrylate and methacrylate monomers including, for example, ethylacrylate, butyl acrylate, n-butyl methacrylate, ethyl methacrylate,methacrylic acid, or combinations thereof. Other acrylate monomers canbe used so long as the desired glass transition temperature, thermalstability, drawability, and refractive index properties are achieved.The ratio of monomers are selected to achieve the desired glasstransition temperature, thermal stability, drawability, and refractiveindex properties. The comonomers can also provide other advantagesincluding, for example, improved interlayer adhesion, increasing ordecreasing the refractive index difference between the first and secondlayers, modifying the melt rheology behavior of the PMMA, or modifyingthe orientation behavior of the PMMA.

As yet another option, second optical layers can be formed using PMMAand a plasticizer that decreases the glass transition temperature of thesecond optical layers. Suitable plasticizers include, for example,phosphoric acid derivatives (e.g., triphenyl phosphate), phthalic acidderivatives (e.g., butyl benzyl phthalate and diisodecyl phthalate),terephthalic acid derivatives (e.g., di-2-ethylhexyl terephthalate),adipic acid derivatives (e.g., polyester adipate), benzoic acidderivatives (e.g., glyceryl tribenzoate), sebacic acid derivatives(e.g., dimethyl sebacate and di-n-butyl sebacate), and acetic acidderivatives (e.g., glyceryl triacetate). A plasticizer can also beselected that improves other properties of the optical body including,for example, interlayer adhesion, increasing or decreasing therefractive index difference between the first and second layers,modifying the melt rheology behavior of the PMMA, or modifying theorientation behavior of the PMMA.

Instead of PMMA, other polymers can be used. For example, other acrylatepolymers with glass transition temperatures lower than PMMA, including,for example, polyethyl methacrylate (PEMA), can be used.

Alternatively, aliphatic polyolefins can be used. Examples of suitablepolyolefins include poly(ethylene-co-octene) (PE-PO),poly(propylene-co-ethylene) (PP-PE), a copolymer of atactic andisotactic polypropylene (aPP-iPP), maleic anhydride functionalizedlinear low density polyethylene (LLDPE-g-MA), and poly(ethylene-co-vinylacetate) (EVA). Other useful polyolefins include acid-modifiedpolyolefins such as “Bynel” polyolefins from E. I. duPont de Nemours &Co., Inc. (Wilmington, Del.), “Primacor” polyolefins from Dow ChemicalCo. (Midland, Mich.), and “Admer” polyolefins from Mitsui PetrochemicalIndustries, Ltd. (Tokyo, Japan). One additional advantage of polyolefinsis that they typically do not substantially degrade at the processingtemperatures utilized with PEN and PET. In addition, the use ofelastomeric polyolefins can enhance mechanical properties of themultilayer optical bodies, including, for example, tear resistance,puncture resistance, and toughness.

Non-Optical Layers

Referring again to FIGS. 1 and 2, one or more of the non-optical layers18 can be formed as a skin layer or skin layers over at least onesurface of stack 16 as illustrated in FIG. 1, to, for example, protectthe optical layers 12, 14 from physical damage during processing and/orafterwards. In addition or alternatively, one or more of the non-opticallayers 18 can be formed within the stack 16 of layers, as illustrated inFIG. 2, to, for example, provide greater mechanical strength to thestack or to protect the stack during processing.

The non-optical layers 18 ideally do not significantly participate inthe determination of optical properties of the multilayer optical film10, at least across the wavelength region of interest (e.g., visible, IRor UV wavelength regions). The non-optical layers 18 may or may not bebirefringent or orientable. Typically, when the non-optical layers 18are used as skin layers there will be at least some surface reflection.In at least some applications where high transmission of light isdesired, the non-optical layers preferably have an index of refractionthat is relatively low (e.g., no more than 1.6 or, preferably, no morethan 1.5) to decrease the amount of surface reflection (e.g.,iridescence). In other applications where reflectivity of light isdesired, the non-optical layers preferably have a relatively highrefractive index (e.g., at least 1.6, more preferably at least 1.7) toincrease reflectance of the multilayer optical body.

When the non-optical layers 18 are found within the stack 16, there willtypically be at least some polarization or reflection of light by thenon-optical layers 18 in combination with the optical layers 12, 14adjacent to the non-optical layers 18. In at least some instances,however, the non-optical layers 18 can be selected to have a thicknessthat dictates that light reflected by the non-optical layers 18 withinthe stack 16 has a wavelength outside the region of interest, forexample, in the infrared region for optical bodies that reflect visiblelight. The thickness of the non-optical layers 18 can be at least twotimes, typically at least four times, and, in many instances, at leastten times, the thickness of one of the individual optical layers 12, 14.The thickness of the non-optical layers 18 can be varied to make anoptical body 10 having a particular thickness. Typically, one or more ofthe non-optical layers 18 are placed so that at least a portion of thelight to be transmitted, polarized, and/or reflected by the opticallayers 12, 14, also travels through the non-optical layers (i.e., thenon-optical layers are placed in the path of light which travels throughor is reflected by the optical layers 12, 14).

The non-optical layers 18 are formed from polymers including any of thepolymer used in the first and second optical layers. In someembodiments, the material selected for the non-optical layers 18 issimilar to or the same as the material selected for the second opticallayers 14. Materials may be chosen for the non-optical layers thatimpart or improve properties such as, for example, tear resistance,puncture resistance, toughness, weatherability, and solvent resistanceof the multilayer optical body.

Other Layers and Coatings

Various functional layers or coatings can be added to the multilayeroptical bodies of the present invention to alter or improve theirphysical or chemical properties, particularly along the surface of themultilayer optical body. Such layers or coatings may include, forexample, slip agents, low adhesion backside materials, conductivelayers, antistatic coatings or films, barrier layers, flame retardants,UV stabilizers, abrasion resistant materials, optical coatings, and/orsubstrates designed to improve the mechanical integrity or strength ofthe film or device, as described in WO 97/01440, which is hereinincorporated by reference. Dichroic polarizing films can also be coatedon or co-extruded with the multilayer optical films, as described, forexample, in WO 95/17691, WO 99/36813, and WO 99/36814, all of which areherein incorporated by reference.

Manufacturing

A brief description of one method for forming multilayer optical bodiesis provided. A filler description of the process conditions andconsiderations is found in PCT Publications Nos. WO 99/36248, WO99/06203, and WO 99/36812, all of which are incorporated herein byreference.

An initial step in the manufacture of the multilayer optical bodies isthe generation of the polymers to be used in formation of the first andsecond optical layers, as well as the non-optical layers (unless thepolymers are available commercially). Typically, these polymers areformed by extrusion, although other methods of polymer formation can beused. Extrusion conditions are chosen to adequately feed, melt, mix andpump the polymer resin feed streams in a continuous and stable manner.Final melt stream temperatures are chosen to be within a range thatreduces freezing, crystallization, or unduly high pressure drops at thelow end of the range and that reduces degradation at the high end of therange. The entire melt stream processing of more than one polymer, up toand including film casting on a chill roll, is often referred to asco-extrusion.

Preferably, the polymers of the first optical layers, the second opticallayers, and the non-optical layers are chosen to have similarrheological properties (e.g., melt viscosities) so that they can beco-extruded. Typically, the second optical layers and the non-opticallayers have a glass transition temperature, Tg, that is either below orno greater than about 30° C. above the glass transition temperature ofthe first optical layers. Preferably, the glass transition temperatureof the second optical layers and the non-optical layers is below theglass transition temperature of the first optical layers.

Following extrusion, each melt stream is conveyed to a gear pump used toregulate the continuous and uniform rate of polymer flow. A staticmixing unit can be used to carry the polymer melt stream from the gearpump into a multilayer feedblock with uniform melt stream temperature.The entire melt stream is typically heated as uniformly as possible toenhance both uniform flow of the melt stream and reduce degradationduring melt processing.

Multilayer feedblocks divide each of the two or more polymer meltstreams into many layers, interleave these layers, and combine the manylayers into a single multilayer stream. The layers from any given meltstream are created by sequentially bleeding off part of the stream froma main flow channel into side channel tubes which lead to layer slots inthe feed block manifold. The layer flow can be controlled by choicesmade in machinery, as well as the shape and physical dimensions of theindividual side channel tubes and layer slots.

The side channel tubes and layer slots of the two or more melt streamsare often interleaved to form alternating layers. The feedblock'sdownstream-side manifold is typically shaped to compress and uniformlyspread the layers of the combined multilayer stack transversely. Thick,non-optical layers, known as protective boundary layers (PBLs), can befed near the manifold walls using the melt streams of the opticalmultilayer stack, or by a separate melt stream. As described above,these non-optical layers can be used to protect the thinner opticallayers from the effects of wall stress and possible resulting flowinstabilities.

The multilayer stack exiting the feedblock manifold enters a finalshaping unit such as a die. Alternatively, the stream can be split,preferably normal to the layers in the stack, to form two or moremultilayer streams that can be recombined by stacking. The stream canalso be split at an angle other than normal to the layers. A flowchanneling system that splits and stacks the streams is called amultiplier. The width of the split streams (i.e., the sum of thethicknesses of the individual layers) can be equal or unequal. Themultiplier ratio is defined as the ratio of the wider to narrower streamwidths. Unequal stream widths (i.e., multiplier ratios greater thanunity) can be useful in creating layer thickness gradients. In the caseof unequal stream widths, the multiplier may spread the narrower streamand/or compress the wider stream transversely to the thickness and flowdirections to ensure matching layer widths upon stacking.

Prior to multiplication, additional non-optical layers can be added tothe multilayer stack. These non-optical layers may perform as PBLswithin the multiplier. After multiplication and stacking, some of theselayers can form internal boundary layers between optical layers, whileothers form skin layers.

After multiplication, the web is directed to a final shaping unit. Theweb is then cast onto a chill roll, sometimes also referred to as acasting wheel or casting drum. This casting is often assisted byelectrostatic pinning, the details of which are well-known in the art ofpolymer film manufacture. The web can be cast to a uniform thicknessacross the web or a deliberate profiling of the web thickness can beinduced using die lip controls.

The multilayer web is then uniaxially or biaxially drawn to produce thefinal multilayer optical film. Uniaxial drawing is performed in a tenteror a length orienter. Biaxial drawing typically includes both types ofequipment. Typical tenters draw in a transverse direction (TD) to theweb path, although certain tenters are equipped with mechanisms to drawor relax (shrink) the film dimensionally in the web path or machinedirection (MD). Length orienters draw in the machine direction.

For example, a two step drawing process is used to orient thebirefringent material in both in-plane directions. The draw processescan be any combination of the single step processes described above thatallow drawing in two in-plane directions. In addition, a tenter thatallows drawing along the machine direction, e.g. a biaxial tenter whichcan draw in two directions sequentially or simultaneously, can be used.In this latter case, a single biaxial draw process can be used.

The following examples demonstrate the manufacture and uses ofmultilayer optical films of the invention. It is to be understood thatthese examples are merely illustrative and are in no way to beinterpreted as limiting the scope of the invention.

EXAMPLES

Monomers, catalysts, and stabilizers utilized in creating polymers forthese examples are commercially available from the following suppliers:dimethyl naphthalene dicarboxylate from Amoco (Decatur, Ala.), dimethylterephthalate from Hoechst Celanese (Dallas, Tex.), ethylene glycol fromUnion Carbide (Charleston, W.Va.), 1,6-hexanediol from BASF (Charlotte,N.C.), antimony triacetate from Elf Atochem (Philadelphia, Pa.),manganese acetate and triethyl phosphonoacetate, both from Albright &Wilson (Glen Allen, Va.).

The polyethylene terephthalate used in the following Examples can besynthesized in a batch reactor with the following raw material charge;5,000 kg dimethyl terephthalate, 3,502 kg ethylene glycol, 1.2 kgmanganese acetate, and 1.6 kg antimony triacetate. Under pressure of1520 torr, this mixture is heated to 254° C. while removing thetransesterification reaction by-product methanol. After 1,649 kg ofmethanol was removed, 2.45 kg of triethyl phosphonoacetate is charged tothe reactor and then the pressure is gradually reduced to 1 torr whileheating to 280° C. The condensation reaction by-product, ethyleneglycol, is continuously removed until a polymer with an IntrinsicViscosity of 0.60, as measured in 60/40 phenol/dichlorobenzene, isproduced.

Example 1

Color-shifting Optical Film with PET:MMA-EA Layers. An optical film wasconstructed with first optical layers created from polyethyleneterephthalate (PET) and second optical layers created from a copolymerusing 75 wt. % methylmethacrylate (MMA) monomers and 25 wt. % of ethylacrylate (EA) monomers. The copolymer is available under the productdesignation “Perspex™ CP63” from ICI Americas, Inc. (Wilmington, Del.).

The above described polymers were then coextruded through a multilayermelt manifold to create a multilayer film with 224 alternating first andsecond optical layers of a total thickness of about 580 μm. Thisparticular multilayer reflective film also contained external protectivelayers made of the same polyethylene terephthalate as the first opticallayers approximately 145 μm thick on each side. The optical film wasinitially preheated, then stretched in the machine direction to a ratioof 3.7:1 at approximately 85° C., and then stretched in the transversedirection to a ratio of 3.9:1 at approximately 95° C. to produce anoptical film of approximately 48 μm thickness.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6433, n_(TD)=1.6757, n_(z)=1.4868). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 4 fornormally incident light. The optical film had a blue appearance thatchanged to red as the viewing angle increased with respect to normalincidence.

Example 2

Color-Shifting Optical Film with PET:MMA-EA Layers. This optical filmwas made in the same way as the optical film of Example 1, except thatduring the drawing process, the optical film was initially preheated to94° C. for 1 minute, then biaxially stretched at a 3.4:3.4 ratio at 94°C. at a draw rate of 20%/s.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6173, n_(TD)=1.6197, n_(z)=1.5108). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 5 fornormally incident light. The optical film had a blue appearance thatchanged to red as the viewing angle increased with respect to normalincidence.

Example 3

Color-Shifting Optical Film with PET:MMA-EA Layers. This optical filmwas made in the same way as the optical film of Example 1, except thatduring the drawing process, the optical film was initially preheated to84° C. for 1 minute, then biaxially stretched at a 2.4:2.4 ratio at 84°C. at a draw rate of 20%/s.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6121, n_(TD)=1.6107, n_(z)=1.5200). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 6 fornormally incident light. The optical film had a red appearance thatchanged to yellow as the viewing angle increased with respect to normalincidence.

Example 4

IR Film with PET:MMA-EA Layers. This optical film was made in the sameway as the optical film of Example 1, except that the total thickness ofthe cast film was 625 μm and during the drawing process, the opticalfilm was initially preheated, then biaxially stretched at a 3.6:3.8ratio at approximately 100° C. The final film thickness was about 51 μm.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6426, n_(TD)=1.6761, n_(z)=1.4896). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 7 fornormally incident light.

Example 5

IR Film with PET:MMA-EA Layers. This optical film was made in the sameway as the optical film of Example 1, except that the total thickness ofthe cast film was 625 μm and during the drawing process, the opticalfilm was initially preheated, then stretched in the machine direction toa ratio of 3.6:1 at approximately 100° C., and then stretched in thetransverse direction to a ratio of 3.8:1 at approximately 100° C. Thefinal film thickness was about 48.8 μm.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6387, n_(TD)=1.6755, n_(z)=1.4945). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 8 fornormally incident light.

Example 6

IR Film with PET:MMA-EA Layers. This optical film was made in the sameway as the optical film of Example 1, except that only 150 alternatinglayers were used, and during the drawing process the optical film wasinitially preheated to 94° C. for 1 minute, then stretched to a drawratio of 3.6:3.2 at 94° C. at a draw rate of 20%/s. The final filmthickness was about 29.5 μm.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6499, n_(TD)=1.6070, n_(z)=1.4969). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 9 fornormally incident light.

Example 7

Color-Shifting and IR Films with PET:MMA-nBMA Layers. Optical films canbe formed using first optical layers created from PET and second opticallayers created from a PMMA copolymer. The PMMA copolymer is formed withMMA comonomer units and n-butyl methacrylate (NBMA) comonomer units. TheNBMA comonomer units depress the T_(g) of the PMMA copolymer below theT_(g) of PMMA.

Example 8

Color-Shifting and IR Films with PET:MMA-nBMA-EA Layers. Optical filmscan be formed using first optical layers created from PET and secondoptical layers created from a PMMA copolymer. The PMMA copolymer isformed with MMA comonomer units, n-butyl methacrylate (NBMA) comonomerunits, and ethyl acrylate (EA) comonomer units. The nBMA and EAcomonomer units depress the T_(g) of the PMMA copolymer below the T_(g)of PMMA. EA comonomer units can also increase the thermal stability ofthe films.

Example 9

Color-Shifting and IR Films with PET:PEMA Layers. Optical films can beformed using first optical layers created from PET and second opticallayers created from polyethyl methacrylate (PEMA). PEMA has a lowerglass transition temperature (67° C.) than PMMA (106° C.).

Example 10

Color-Shifting and IR Films with PET:EMA-BA Layers. Optical films can beformed using first optical layers created from PET and second opticallayers created from a PEMA copolymer. The PEMA copolymer is formed withethyl methacrylate (EMA) comonomer units and butyl acrylate (BA)comonomer units. The butyl acrylate comonomer units have a higherallowable odor threshold than ethyl methacrylate and so can be used tomake a more acceptable polymer layer.

Example 11

Color-Shifting and IR Films with PET:MMA-EA-MA Layers. Optical films canbe formed using first optical layers created from PET and second opticallayers created from a PMMA copolymer. The PMMA copolymer is formed withMMA comonomer units, ethyl acrylate (EA) comonomer units, andmethacrylic acid (MA) comonomer units. The EA comonomer units depressthe T_(g) of the PMMA copolymer below the T_(g) of PMMA. EA comonomerunits can also increase the thermal stability of the film. MA comonomerunits are used to increase interlayer adhesion with the PET firstoptical layers.

Example 12

Color-Shifting Films with PET:PMMA/PVDF (75/25) Layers. Optical filmscan be formed using first optical layers created from PET and secondoptical layers created from a blend of PMMA and poly(vinylidenefluoride) (PVDF) (e.g., Solef™ 1008 available from Solvay Polymers, Inc.(Houston, Tex.)) in a ratio of 75/25 by weight. The PMMA/PVDF blend as alower T_(g) (72° C.) and refractive index (1.47) than PMMA. Therefractive index difference between the first and second optical layersis about 0.18. PVDF also provides other benefits including improvedsolvent resistance and weatherability.

Example 13

Color-Shifting and IR Films with PET:PMMA/PVDF (60/40) Layers. Opticalfilms can be formed using first optical layers created from PET andsecond optical layers created from a blend of PMMA and poly(vinylidenefluoride) (PVDF) (e.g., Solef™ 1008 available from Solvay Polymers, Inc.(Houston, Tex.)) in a ratio of 60/40 by weight. The PMMA/PVDF blend hasa lower T_(g) (66° C.) and refractive index (1.458) than PMMA. Therefractive index difference between the first and second optical layersis about 0.19. PVDF also provides other benefits including improvedsolvent resistance and weatherability.

Example 14

IR Film with PET:PMMA/PVDF (75/25) Layers. An optical film wasconstructed with first optical layers created from a polyethyleneterephthalate (PET) and second optical layers created from a blend using75 wt. % PMMA (Perspex™ CP82, ICI Americas, Inc. (Wilmington, Del.)) and25 wt. % polyvinylidene fluoride (PVDF) (Solef™ 1008, Solvay Polymers,Inc. (Houston, Tex.)).

The above described polymers were then coextruded through a multilayermelt manifold to create a multilayer film with 196 alternating first andsecond optical layers. This particular multilayer reflective film alsocontained external protective layers made of the same polyethyleneterephthalate as the first optical layers. The optical film wasinitially preheated, then stretched to a ratio of between 3.6 and 3.8 ineach of the machine and transverse directions at approximately 100° C.to produce an optical film of approximately 40.4 μm thick.

The optical film had the transmission spectrum illustrated in FIG. 10for normally incident light.

Example 15

Color-shifting Optical Film with PET:PE-PO Layers. An optical film wasconstructed with first optical layers created from a polyethyleneterephthalate (PET) and second optical layers created from a polyolefincopolymer poly(ethylene-co-octene) (PE-PO). The copolymer is availableunder the product designation “Engage 8200” from Dow-DuPont Elastomers.

The above described polymers were then coextruded through a multilayermelt manifold to create a multilayer film with 224 alternating first andsecond optical layers. The cast film had a total thickness of about 533μm. This particular multilayer reflective film also contained externalprotective layers made of the same polyethylene terephthalate as thefirst optical layers. The optical film was initially preheated, thenstretched in the machine direction to a ratio of 3.8:1 at approximately100° C., and then stretched in the transverse direction to a ratio of3.6:1 at approximately 100° C. to produce an optical film ofapproximately 38.6 μm.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6766, n_(TD)=1.6400, n_(z)=1.4906). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 11for normally incident light. The optical film had a blue appearance thatchanged to red as the viewing angle increased with respect to normalincidence.

Example 16

IR Film with PET:PE-PO Layers. An optical film was constructed withfirst optical layers created from a polyethylene terephthalate (PET) andsecond optical layers created from a polyolefin copolymerpoly(ethylene-co-octene) (PE-PO). The copolymer is available under theproduct designation “Engage 8200” from Dow-DuPont Elastomers.

The above described polymers were then coextruded through a multilayermelt manifold to create a multilayer film with 224 alternating first andsecond optical layers. The cast film had a total thickness of about 533μm. This particular multilayer reflective film also contained externalprotective layers made of the same polyethylene terephthalate as thefirst optical layers. The optical film was initially preheated, thenstretched in the machine direction to a ratio of 3.6:1 at approximately100° C., and then stretched in the transverse direction to a ratio of3.8:1 at approximately 100° C. to produce an optical film ofapproximately 57.1 μm.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6400, n_(TD)=1.6766, n_(z)=1.4906). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 12for normally incident light. The optical film had a clear appearancewith a slight yellow observed at higher viewing angles.

Example 17

Color-shifting Optical Film with PET:PP-PE Layers. An optical film wasconstructed with first optical layers created from a polyethyleneterephthalate (PET) and second optical layers created from a polyolefincopolymer poly(propylene-co-ethylene) (PP-PE). The copolymer isavailable under the product designation “Z9470” from Fina Oil andChemical Co., Dallas, Tex.

The above described polymers were then coextruded through a multilayermelt manifold to create a multilayer film with 224 alternating first andsecond optical layers. The cast film had a total thickness of about 719μm. This particular multilayer reflective film also contained externalprotective layers made of the same polyethylene terephthalate as thefirst optical layers. The cast film was heated in an oven charged withhot air set at 94° C. and then oriented at a 4.4:4.4 draw at a drawtemperature of 94° C. and a draw rate of 20%/s.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6208, n_(TD)=1.6164, n_(z)=1.5132). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 13for normally incident light. The optical film had a blue appearance thatchanged to red as the viewing angle increased with respect to normalincidence.

Example 18

IR Film with PET:PP-PE Layers. An optical film was constructed withfirst optical layers created from a polyethylene terephthalate (PET) andsecond optical layers created from a polyolefin copolymerpoly(propylene-co-ethylene) (PP-PE). The copolymer is available underthe product designation “Z9470” from Fina Oil and Chemical Co., Dallas,Tex.

The above described polymers were then coextruded through a multilayermelt manifold to create a multilayer film with 224 alternating first andsecond optical layers. The cast film had a total thickness of about 719μm. This particular multilayer reflective film also contained externalprotective layers made of the same polyethylene terephthalate as thefirst optical layers. The optical film was initially preheated, thenstretched in the machine direction to a ratio of 3.6:1 at approximately100° C., and then stretched in the transverse direction to a ratio of3.8:1 at approximately 100° C. to produce an optical film ofapproximately 42.4 μm.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6376, n_(TD)=1.6852, n_(z)=1.4860). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 14for normally incident light. The optical film had a slight yellowappearance at normal incidence, but became clear as viewing angleincreased.

Example 19

Color-shifting Optical Film with PET:aPP-iPP Layers. An optical film wasconstructed with first optical layers created from a polyethyleneterephthalate (PET) and second optical layers created from a polyolefincopolymer of atactic polypropylene (aPP) and isotactic polypropylene(iPP). The copolymer is available under the product designation “RexflexW111” from Huntsman Chemical Corp., Salt Lake City, Utah.

The above described polymers were then coextruded through a multilayermelt manifold to create a multilayer film with 224 alternating first andsecond optical layers. The cast film had a total thickness of about 683μm. This particular multilayer reflective film also contained externalprotective layers made of the same polyethylene terephthalate as thefirst optical layers. The cast film was heated in an oven charged withhot air set at 94° C. and then oriented at a 3.5:3.5 draw at a drawtemperature of 94° C. and a draw rate of 20%/s to produce an opticalfilm of approximately 34 μm.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6201, n_(TD)=1.6206, n_(z)=1.5064). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 15for normally incident light. The optical film had a blue appearance thatchanged to red as the viewing angle increased with respect to normalincidence.

Example 20

IR Film with PET:aPP-iPP Layers. An optical film was constructed withfirst optical layers created from a polyethylene terephthalate (PET) andsecond optical layers created from a polyolefin copolymer of atacticpolypropylene (aPP) and isotactic polypropylene (iPP). The copolymer isavailable under the product designation “Rexflex W111” from HuntsmanChemical Corp., Salt Lake City, Utah.

The above described polymers were then coextruded through a multilayermelt manifold to create a multilayer film with 224 alternating first andsecond optical layers. The cast film had a total thickness of about 683μm. This particular multilayer reflective film also contained externalprotective layers made of the same polyethylene terephthalate as thefirst optical layers. The cast film was heated in an oven charged withhot air set at 94° C. and then oriented at a 2.8:2.8 draw at a drawtemperature of 94° C. and a draw rate of 20%/s to produce an opticalfilm.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6214, n_(TD)=1.6199, n_(z)=1.5059). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 16for normally incident light. The optical film had a slight red tint atnormal incidence, but became clear as viewing angle increased.

Example 21

Color-shifting Optical Film with PET:LLDPE-g-MA Layers. An optical filmwas constructed with first optical layers created from a polyethyleneterephthalate (PET) and second optical layers created from afunctionalized polyolefin, linear low density polyethylene-g-maleicanhydride (LLDPE-g-MA). The copolymer is available under the productdesignation “Bynel 4105” from E. I. duPont de Nemours & Co., Inc.(Wilmington, Del.).

The above described polymers were then coextruded through a multilayermelt manifold to create a multilayer film with 224 alternating first andsecond optical layers. This particular multilayer reflective film alsocontained external protective layers made of the same polyethyleneterephthalate as the first optical layers. The cast film was heated inan oven charged with hot air set at 94° C. and then oriented at a3.5:3.7 draw at a draw temperature of 94° C. and a draw rate of 20%/s toproduce an optical film of approximately 33.8 μm.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6175, n_(TD)=1.6268, n_(z)=1.5075). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 17for normally incident light.

Example 22

IR Film with PET:LLDPE-g-MA Layers. An optical film was constructed withfirst optical layers created from a polyethylene terephthalate (PET) andsecond optical layers created from a functionalized polyolefin, linearlow density polyethylene-g-maleic anhydride (LLDPE-g-MA). The copolymeris available under the product designation “Bynel 4105” from E. I.duPont de Nemours & Co., Inc. (Wilmington, Del.).

The above described polymers were then coextruded through a multilayermelt manifold to create a multilayer film with 224 alternating first andsecond optical layers. This particular multilayer reflective film alsocontained external protective layers made of the same polyethyleneterephthalate as the first optical layers. The optical film wasinitially preheated, then stretched in the machine direction to a ratioof 3.6:1 at approximately 100° C., and then stretched in the transversedirection to a ratio of 3.8:1 approximately 100° C. to produce anoptical film of approximately 40.9 μm.

The PET first optical layers were oriented by the process as determinedby refractive index measurement of the external protective PET layers(n_(MD)=1.6378, n_(TD)=1.6847, n_(z)=1.4869). The second optical layerswere essentially isotropic with a refractive index of about 1.49.

The optical film had the transmission spectrum illustrated in FIG. 18for normally incident light. The optical film was clear at normalincidence, but became slightly blue as viewing angle increased.

Example 23

Color-Shifting and IR Films with PET:EVA Layers. Oriented Optical filmscan be formed using first optical layers created from PET and secondoptical layers created from poly(ethylene-co-vinyl acetate) (EVA).

Example 24

Color-Shifting and IR Films with PET:EVA/PE-PP Layers. Oriented Opticalfilms can be formed using first optical layers created from PET andsecond optical layers created from a miscible blend ofpoly(ethylene-co-vinyl acetate) and poly(ethylene-co-propylene).

Example 25

Color-Shifting and IR Films with PET-PC:PMMA Layers. Oriented Opticalfilms can be formed using first optical layers created fromcopolymerization or reactive blending of PET and polycarbonate (PC) andsecond optical layers created from polymethyl methacrylate (PMMA).Polycarbonate has a glass transition temperature of 157° C.

Example 26

Color-Shifting and IR Films with PET-PAr:PMMA Layers. Oriented Opticalfilms can be formed using first optical layers created fromcopolymerization or reactive blending of PET and polyarylate (PAr) andsecond optical layers created from polymethyl methacrylate (PMMA).Polyarylate has a glass transition temperature of 193° C.

Example 27

Color-Shifting and IR Films with PET/PEI:PMMA Layers. Oriented Opticalfilms can be formed using first optical layers created from miscibleblending of PET and polyetherimide (PEI) and second optical layerscreated from polymethyl methacrylate (PMMA). Polyetherimide has a glasstransition temperature of 218° C.

Example 28

Color-Shifting and IR Films with PET-PEN (90/10):EVA Layers. OrientedOptical films can be formed using first optical layers created fromcopolymerization or reactive blending of 90 wt. % PET and 10 wt. % PENand second optical layers created from poly(ethylene-co-vinyl acetate)(EVA).

Example 29

Color-Shifting and IR Films with PET-PEN (30/70):EVA Layers. OrientedOptical films can be formed using first optical layers created fromcopolymerization or reactive blending of 30 wt. % PET and 70 wt. % PENand second optical layers created from poly(ethylene-co-vinyl acetate)(EVA).

Examples 30

Multilayer Optical Film with PEN:PMMA/PVDF(60/40) Layers

A multilayer cast film was coextruded containing about 450 layers ofalternating PEN (0.48 intrinsic viscosity from Eastman ChemicalProducts, Inc., Kingsport, Tenn.) and a miscible PMMA/PVDF(60/40, byweight) blend. PMMA was Perspex™ CP80 from ICI Americas, Inc.(Wilmington, Del.) and PVDF was Solef™ 1008 from Solvay Polymers, Inc.(Houston, Tex.). The extruded cast film was subsequently andcontinuously biaxally-oriented. The sequential biaxial orientationprocess involved a first orientation step in the machine direction (MD)in a length orienter followed by a transverse direction (TD) orientationin a tenter. The finished optical film had an overall thickness ofapproximately 58.4 μm.

For comparison a PEN:PMMA multilayer mirror film was processed with thesame equipment and under similar conditions.

The two films differed in the refractive index of the low indexmaterial, PMMA (n=1.49) and PMMA/PVDF(60/40) (n=1.458). As stated, thehigh index material is the same for both films, PEN (n=1.75).

FIG. 19 clearly shows the higher average reflectivity of thePEN:PMMA/PVDF(60/40) (Δn=0.29) multilayer mirror film relative to thePEN:PMMA (Δn=0.26) over a majority of the width. The reflectivity ofFIG. 1 is a photopical average from measured transmission spectra (CIEIlluminant C, 2° observer).

FIG. 20 shows the reflectivity of the PEN:PMMA and PEN:PMMA/PVDF(60/40)films as a function of wavelength. The reflectivity in FIG. 20 iscalculated from measured transmission (% reflectivity=100−%transmission). The PEN:PMMA film shows a relatively nonuniform spectrumwith a significant degree of “spikes” which lead to undesirable spectralnonuniformity (iridescence). The spectral nonuniformity (iridescence) isassociated with processing difficulties including layer thicknessvariations and lack of optical packet overlap. The higher refractiveindex difference of the PEN:PMMA/PVDF(60/40) multilayer mirror filmresults in a significant dampening of the spectral nonuniformity(iridescence). The demonstrated reduction in spectral nonuniformity(iridescence) in the PEN:PMMA/PVDF(60/40) film relative to the PEN:PMMAfilm represents both an optical and processing benefit.

FIG. 21 shows the same reduction in spectral nonuniformity (iridescence)with PEN:PMMA/PVDF(60/40) relative to PEN:PMMA for films of similarbandwidths.

FIGS. 22 and 23 show a comparison of color index parameters as afunction of crossweb distance for the PEN:PMMA and PEN:PMMA/PVDF(60/40)films. The color index parameters were photopically averaged frommeasured transmission spectra (CIE Illuminant C, 2° observer). Ideallythe color index parameters are zero. The figures showPEN:PMMA/PVDF(60/40) film shows better color performance relative to thePEN:PMMA film over a majority of the film width.

The crossweb caliper uniformity of the PEN:PMMA film was significantlybetter than the PEN:PMMA/PVDF(60/40) film. It is noted that the timeinvested to achieve good crossweb caliper uniformity in the PEN:PMMAfilm was not invested to achieve the same in the PEN:PMMA/PVDF(60/40)film. Despite the better crossweb caliper uniformity of the PEN:PMMAfilm, FIGS. 19-23 show superior optical performance of thePEN:PMMA/PVDF(60/40) film. This result illustrates a relaxation of thecrossweb caliper uniformity constraint associated with processing ofmultilayer optical films. This is another example of how a largerefractive index difference material combination such asPEN:PMMA/PVDF(60/40) results in a more robust process. Furthermore, itis expected that better crossweb caliper uniformity is achievable in thePEN:PMMA/PVDF(60/40) film resulting in even better optical performancerelative to a PEN:PMMA film.

Example 31

Multilayer Optical Film with coPEN(70/30):PMMA PVDF(60/40) Layers. Amultilayer optical film similar to that of Example 30 was formed exceptthat a copolymer of PEN with carboxylate subunits formed using 70 wt. %naphthalate monomers and 30 wt. % terephthalate monomers was used. Thefirst optical layers of this multilayer optical film possess the samerefractive index difference as the PEN:PMMA formulation (Δn=0.29) thusthe expectation of similar optical and processing performance. The lowerPEN content provides a film with a UV absorption band edge further fromthe visible region of the spectrum allowing easier UV protection of thefilm. The lower glass transition temperature of the coPEN (110° C.)relative to PEN (124° C.) may also have benefits in thermoforming,embossing, and the like.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the instant specification.

1. An optical body, comprising: a plurality of first optical layers,each first optical layer being oriented and comprising a polyesterhaving terephthalate comonomer units and ethylene glycol comonomerunits; and a plurality of second optical layers disposed in a repeatingsequence with the plurality of first optical layers, each second opticallayer comprising a copolymer of polymethyl methacrylate that containscomonomer units that depress a glass transition temperature of thecopolymer below a glass transition temperature of the polyester of thefirst optical layers; the optical body being configured and arranged toreflect at least a portion of light over at least one wavelength region.2. The optical body of claim 1, wherein the comonomer units are selectedfrom ethyl acrylate, butyl acrylate, n-butyl methacrylate, and ethylmethacrylate.
 3. The optical body of claim 1, wherein the first opticallayers have an in-plane birefringence of at least about 0.05.
 4. Theoptical body of claim 1, wherein at least one in-plane index ofrefraction of the first optical layers differs by at least about 0.1from an in-plane index of refraction, in the same direction, of thesecond optical layers.
 5. The optical body of claim 1, wherein theoptical body is configured and arranged to reflect at least asubstantial portion of light in a visible wavelength region.
 6. Theoptical body of claim 5, wherein the optical body has a blue appearancethat shifts to red as the viewing angle increases with respect to normalincidence.
 7. The optical body of claim 1, wherein the optical body isconfigured and arranged to reflect a substantial portion of light in aninfrared wavelength region.
 8. The optical body of claim 1, wherein thefirst polyester is polyethylene terephthalate.
 9. The optical body ofclaim 1, wherein the first and second optical layers are coextruded. 10.The optical body of claim 1, wherein the first optical layers arebiaxially oriented.
 11. The optical body of claim 1, wherein z-axisrefractive indices of the first and second optical layers differ by nomore than about 0.04.
 12. An article, comprising: a mirror comprising aplurality of first optical layers, each first optical layer beingoriented and comprising a polyester having terephthalate comonomer unitsand ethylene glycol comonomer units, and a plurality of second opticallayers disposed in a repeating sequence with the plurality of firstoptical layers, each second optical layer comprising a copolymer ofpolymethyl methacrylate that contains comonomer units that depress aglass transition temperature of the copolymer below a glass transitiontemperature of the polyester of the first optical layers, the mirrorbeing configured and arranged to reflect at least a portion of lightover at least one wavelength region.
 13. The article of claim 12,wherein the mirror is configured and arranged to reflect a substantialportion of light in an infrared wavelength region.
 14. The article ofclaim 12, wherein the mirror is configured and arranged to reflect asubstantial portion of light in a visible wavelength region.
 15. Thearticle of claim 12, wherein the mirror is configured and arranged toreflect a substantial portion of light in an ultraviolet wavelengthregion.
 16. An article, comprising: a light source; and an optical bodydisposed to receive light from the light source, the optical bodycomprising a plurality of first optical layers, each first optical layerbeing oriented and comprising a polyester having terephthalate comonomerunits and ethylene glycol comonomer units, and a plurality of secondoptical layers disposed in a repeating sequence with the plurality offirst optical layers, each second optical layer comprising a copolymerof polymethyl methacrylate that contains comonomer units that depress aglass transition temperature of the copolymer below a glass transitiontemperature of the polyester of the first optical layers, the opticalbody being configured and arranged to reflect at least a portion oflight over at least one wavelength region.
 17. The article of claim 16,wherein the optical body is configured and arranged to reflect asubstantial portion of light in an infrared wavelength region.
 18. Thearticle of claim 16, wherein the optical body is configured and arrangedto reflect a substantial portion of light in a visible wavelengthregion.
 19. The article of claim 16, wherein the optical body isconfigured and arranged to reflect a substantial portion of light in anultraviolet wavelength region.
 20. The article of claim 16, whereinz-axis refractive indices of the first and second optical layers differby no more than about 0.04.